Thin film transistor, method of producing the same, liquid crystal display, and thin film forming apparatus

ABSTRACT

The invention intends to provide a TFT having a gate insulating film which has a high dielectric withstand voltage and can ensure a desired carrier mobility in an adjacent semiconductor active film. A gate electrode and a semiconductor active film are formed on a transparent substrate with a gate insulating film, which is formed of two layered insulating films, held between them. The gate insulating film is made up of a first gate insulating film which improves a withstand voltage between the gate electrode and the semiconductor active film, and a second gate insulating film which improves an interface characteristic between the gate insulating film and the semiconductor active film . The first and second gate insulating films are each formed of a SiN x  film. The optical band gap of the first gate insulating film has a value in the range of 3.0 to 4.5 eV, and the optical band gap of the second gate insulating film has a value in the range of 5.0 to 5.3 eV.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin film transistor, a method ofproducing the thin film transistor, a liquid crystal display, and a thinfilm forming apparatus. More particularly, the present invention relatesto the structure of a gate insulating film in a thin film transistor ofthe reverse stagger type.

2. Description of the Related Art

FIG. 14 relates to a conventional ordinary liquid crystal display usingthin film transistors (hereinafter referred to as TFTS), and illustratesone example of the structure of a TFT array board including TFTs of thereverse stagger type, gate lines, source lines, etc. In such a TFT arrayboard, as shown in FIG. 14, gate lines 50 and source lines 51 arearranged on a transparent substrate in a matrix pattern. Each of. areassurrounded by the gate lines 50 and the source lines 51 serves as onepixel 52, and a TFT 53 is provided for each pixel 52. FIG. 15 is asectional view showing a construction of the TFT 53.

In the TFT 53, as shown in FIG. 15, a gate electrode 55 leading out ofthe gate line 50 is formed on a transparent substrate 54, and a gateinsulating film 56 is formed in covering relation to the gate electrode55. A semiconductor active film 57 made of amorphous silicon (a-Si) isformed on the gate insulating film 56 at a position above the gateelectrode 55. A source electrode 59 leads out of the source line 51 anda drain electrode 60 are formed to extend over the semiconductor activefilm 57 through an ohmic contact layer 58 which is made of amorphoussilicon (a-Si:n⁺) containing an n-type impurity such as phosphorous, andthen on the gate insulating film 56. A passivation film 61 is formed incovering relation to the TFT 53 made up of the source electrode 59, thedrain electrode 60, the gate electrode 55, etc., and a contact hole 62is formed in the passivation film 61 at a position above the drainelectrode 60. Further, a pixel electrode 63 formed of a transparentconductive film, such as indium tin oxide (hereinafter referred to asITO), is filled in the contact hole 62 for electrical connection to thedrain electrode 60.

Of the components of the TFT thus constructed, the gate insulating filmlocated between the gate electrode and the semiconductor active film isthe most important component that dominates electrical characteristicsand reliability of the TFT. Also, the gate insulating film is an elementthat is responsible for the occurrence of surface defects. For anamorphous-silicon TFT using amorphous silicon as a material of thesemiconductor active film, a redundant structure endurable againstdefects has been tried by employing a two-layered gate insulating filmstructure wherein gate insulating films are formed as two stacked layersusing different materials and different methods. In one example of sucha structure, the two stacked layers are a dense film of Ta₂O₅ formed byanode-oxidizing tantalum (Ta) of the gate electrode and a film of Si₃N₄deposited by the plasma CVD.

Regarding electrical characteristics of the TFT, generally-demandedcapabilities of the gate insulating film are represented by a dielectricwithstand voltage and a carrier mobility in the semiconductor activefilm. The dielectric withstand voltage is a problem inherently dependingon the gate insulating film itself, whereas the carrier mobility in thesemiconductor active film is affected by an interface characteristicbetween the gate insulating film and the semiconductor active film.

The term “dielectric withstand voltage” means a maximum voltage untilwhich the gate insulating film is endurable against dielectric breakdownin a test wherein the voltage applied between the gate electrode and thesemiconductor active film is increased gradually. If the dielectricwithstand voltage is lower than a desired design value, the gateinsulating film would be liable to break down, thus resulting in anoperation failure of the TFT and hence a display failure.

Also, the term “mobility” means an index indicating easiness in movementof carries within the TFT. A larger value of the mobility represents agreater driving ability and a higher-speed operation of the TFT. Themobility lowers if traveling of carriers is impeded due to disorder of asemiconductor crystal and the presence of impurities. Taking electronsin silicon as an example, the mobility of electrons is about 1000cm²/V·sec in a single crystal. However, the mobility lowers down to theorder of 0-100 cm²/V·sec in polycrystalline silicon, and further down tothe order of 0.3-1 cm²/V·sec in amorphous silicon. In other words,because the mobility lowers in the case of using amorphous silicon dueto the inherent property, there has been a demand for maintaining themobility as high as possible even to a small extent in such a case.

Although the dielectric withstand voltage and the carrier mobility are,as described above, important factors in achieving TFTs with goodelectrical characteristics and high reliability, the materials whichhave been usually employed for the gate insulating film in the past arenot satisfactory from points of both the dielectric withstand voltageand the carrier mobility. Also, although it has been hitherto proposedto combine two kinds of layers for giving the gate insulating filmdesired capabilities like the above-mentioned example of the two-layeredstructure of Ta₂O₅ and Si₃N₄, this method has such problems that thestep of forming the gate insulating film is complicated and theproductivity of TFT array boards is deteriorated.

SUMMARY OF THE INVENTION

With the view of solving the problems set forth above, an object of thepresent invention is to provide a TFT having a gate insulating filmwhich has a high dielectric withstand voltage and can ensure a desiredcarrier mobility in an adjacent semiconductor active film, a method ofproducing the TFT, a liquid crystal display which is superior inelectrical characteristics and yield, as well as a thin film formingapparatus adaptable in the method of producing the TFT.

To achieve the above object, in the TFT of the present invention, a gateelectrode and a semiconductor active film are formed on a substrate witha gate insulating film, which is formed of two layered insulating films,located therebetween, the gate insulating film being made up of a firstgate insulating film which is disposed on the same side as the gateelectrode and improves a withstand voltage between the gate electrodeand the semiconductor active film, and a second gate insulating filmwhich is disposed on the same side as the semiconductor active film andimproves an interface characteristic between the gate insulating filmand the semiconductor active film.

In other words, the TFT of the present invention intends to realize agate insulating film which has in itself a desired dielectric withstandvoltage and renders the semiconductor active film to have a desiredcarrier mobility, by forming the gate insulating film with two layeredinsulating films made of such materials as functioning respectively toimprove the withstand voltage between the gate electrode and thesemiconductor active film, and to improve the interface characteristicbetween the gate insulating film and the semiconductor active film. Thephrase “improve the interface characteristic between the gate insulatingfilm and the semiconductor active film” used herein means that thecarrier mobility in the semiconductor active film is improved as aresult of forming the second gate insulating film.

Concrete examples of the materials usable as the first and second gateinsulating films are as follows. The first and second gate insulatingfilms are each formed of a silicon nitride film, the optical band gap ofthe first gate insulating film has a value in the range of 3.0 to 4.5eV, and the optical band gap of the second gate insulating film has avalue in the range of 5.0 to 5.3 eV.

Heretofore, it has been customary that an insulating film having twodifferent functions is formed of two layered films by using twodifferent kinds of materials. However, the inventor has found that evenfilms having the same composition, i.e., silicon nitride films, developdifferent characteristics if values of the optical band gap of the filmsare different from each other, and has accomplished the presentinvention based on the finding. The correlation between concretenumerical values of the optical band gap and characteristics of the gateinsulating film will be described later in Examples.

A method of producing the TFT of the present invention, which includes aprocess of forming the gate insulating film having the above features,comprises the steps of preparing a plasma CVD apparatus including aradio-frequency electrode and a susceptor electrode disposed in opposedrelation and installed in a film forming chamber; bringing a gas mixtureof silane gas and ammonia gas into a plasma state under a desiredradio-frequency electric field formed between the radio-frequencyelectrode and the susceptor electrode, thereby forming a first gateinsulating film on a gate electrode formed on a substrate; bringing agas mixture having the same composition as the above gas mixture into aplasma state under a greater radio-frequency electric field than theabove radio-frequency electric field, thereby forming a second gateinsulating film on the first gate insulating film; and forming asemiconductor active film on the second gate insulating film.

When the above method is employed, radio-frequency powers can be appliedto the radio-frequency electrode and the susceptor electrode of theplasma CVD apparatus in any sequence of the following combinations. Itis to be here noted that the power applied to the radio-frequencyelectrode is called excitation power and the power applied to thesusceptor electrode is called substrate bias power.

(1) The substrate bias power is not applied both in forming the firstgate insulating film and in forming the second gate insulating film,whereas the excitation power is set to be greater in forming the secondgate insulating film than in forming the first gate insulating film.

(2) The equal substrate bias power is applied both in forming the firstgate insulating film and in forming the second gate insulating film,whereas the excitation power is set to be greater in forming the secondgate insulating film than in forming the first gate insulating film.

(3) The substrate bias power is set to be greater in forming the secondgate insulating film than in forming the first gate insulating film,whereas the equal excitation power is applied both in forming the firstgate insulating film than in forming the second gate insulating film.

(4) The substrate bias power is set to be greater in forming the secondgate insulating film than in forming the first gate insulating film,whereas the excitation power is set to be greater in forming the secondgate insulating film than in forming the first gate insulating film.

Of the above combinations, the cases of applying both the substrate biaspower and the excitation power require the use of a two-frequencyexcitation plasma CVD apparatus.

Instead of the above method of using a gas mixture having the samecomposition as the gas mixture used in forming the first gate insulatingfilm and forming the second gate insulating film under a greaterradio-frequency electric field than in forming the first gate insulatingfilm, the method of producing the TFT may comprise the steps of formingthe first gate insulating film in the same manner as in the abovemethod; bringing a gas mixture, in which silane gas and ammonia gas aremixed at such a mixing ratio as containing the ammonia gas at a greaterproportion relative to the silane gas than in the mixture gas used inthe preceding step, into a plasma state under a radio-frequency electricfield having the same intensity as the radio-frequency electric fieldapplied in forming the first gate insulating film, thereby forming asecond gate insulating film on the first gate insulating film; andforming a semiconductor active film on the second gate insulating film.

Also, when the above method of changing over the composition of the gasmixture is employed, radio-frequency powers can be applied to theradio-frequency electrode and the susceptor electrode of the plasma CVDapparatus in any sequence of the above combinations (1) to (4).

With any of the above methods, the two layered gate insulating filmshaving different characteristics can be formed successively and easilyjust by forming both the first and second gate insulating films ofsilicon nitride films, and changing over the radio-frequency power orthe composition of the gas mixture at the time of shift from the firstgate insulating film forming step to the second gate insulating filmforming step. As a result, the step of forming the gate insulating filmis not so complicated as the example of the two-layered structure ofTa₂O₅ and Si₃N₄ mentioned above in connection with the related art, anda TFT array board can be manufactured with productivity at a level notso lower than the case of forming a one-layer gate insulating film.

In the liquid crystal display of the present invention, a liquid crystalis held between a pair of substrates disposed in opposed relation, andone of the pair of substrates includes the above TFT.

The liquid crystal display of the present invention employs a TFT arrayboard having the TFTs which provide a high dielectric withstand voltagebetween the gate electrode and the semiconductor active film, and ensurea great carrier mobility in the semiconductor active film. Therefore, aliquid crystal display can be realized which has a high response speedand is superior in yield and reliability.

The thin film forming apparatus of the present invention comprises asusceptor electrode disposed in opposed relation to a radio-frequencyelectrode and installed in a film forming chamber for supporting asubstrate thereon, and a control unit for successively carrying out thesteps of supplying a reactive gas to an inner space of the film formingchamber while exhausting the gas so as to maintain a desired pressurewithin the film forming chamber, and bringing the reactive gas into aplasma state under a first radio-frequency electric field formed betweenthe radio-frequency electrode and the susceptor electrode, therebyforming a first coating on the substrate; and bringing the reactive gasinto a plasma state under a greater second radio-frequency electricfield than the first radio-frequency electric field while maintainingthe plasma state between the radio-frequency electrode and the susceptorelectrode, thereby forming a second coating on the surface of the firstcoating.

As means for producing the second radio-frequency electric field greaterthan the first radio-frequency electric field, second substrate biaspower applied to the susceptor electrode in forming the second coatingmay be set to be greater than first substrate bias power applied to thesusceptor electrode in forming the first coating, while desired plasmaexcitation power is applied to the radio-frequency electrode.Alternatively, second plasma excitation power applied to theradio-frequency electrode in forming the second coating may be set to begreater than first plasma excitation power applied to theradio-frequency electrode in forming the first coating.

Another thin film forming apparatus of the present invention comprises asusceptor electrode disposed in opposed relation to a radio-frequencyelectrode and installed in a film forming chamber for supporting asubstrate thereon, and a control unit for successively carrying out thesteps of supplying a first gas mixture of monosilane gas and ammoniagas, which are mixed at a first mixing ratio, to an inner space of thefilm forming chamber while exhausting the gas so as to maintain adesired pressure within the film forming chamber; bringing the first gasmixture into a plasma state under a radio-frequency electric fieldformed between the radio-frequency electrode and the susceptorelectrode, thereby forming a first silicon nitride film on thesubstrate; and supplying a second gas mixture of monosilane gas andammonia gas, which are mixed at such a second mixing ratio as containingthe ammonia gas at a greater proportion than at the first mixing ratio,to the inner space of the film forming chamber while maintaining theplasma state between the radio-frequency electrode and the susceptorelectrode, and bringing the second gas mixture into a plasma state tothereby form a second silicon nitride film on the surface of the firstsilicon nitride film.

With the thin film forming apparatus of the present invention, the twolayered coatings having different characteristics can be formedsuccessively in the single apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the structure of a TFT according toone embodiment of the present invention;

FIG. 2 shows a process flow; namely, FIGS. 2A to 2E show successivesteps of producing the TFT according to the embodiment;

FIG. 3 is a schematic view showing a two-frequency excitation plasma CVDapparatus used in the embodiment;

FIG. 4 is a flowchart showing a first sequence of the process carriedout by the plasma CVD apparatus;

FIG. 5 is a flowchart showing a second sequence of the process carriedout by the plasma CVD apparatus;

FIG. 6 is a chart for explaining a sequence of forming an gateinsulating film of the TFT, the graph showing an example of a manner ofchanging substrate bias power;

FIG. 7 is a chart for explaining a sequence of forming the gateinsulating film of the TFT, the graph showing an example of a manner ofchanging RF excitation power;

FIG. 8 is a chart for explaining a sequence of forming the gateinsulating film of the TFT, the graph showing an example of a manner ofchanging a flow rate ratio of mixed gases;

FIG. 9 is a sectional view showing the schematic construction of aliquid crystal display employing a TFT array board provided with theTFTs according to the embodiment;

FIG. 10 is a graph of experimental data showing the correlation betweenthe substrate bias power and the optical band gap in a process of filmformation according to the embodiment of the present invention;

FIG. 11 is a graph of experimental data showing the correlation betweenthe applied RF power and the flat-portion dielectric withstand voltagein the process of film formation according to the embodiment of thepresent invention;

FIG. 12 is a graph of experimental data showing the correlation betweenthe applied RF power and the stepped-portion dielectric withstandvoltage in the process of film formation according to the embodiment ofthe present invention;

FIG. 13 is a graph of experimental data showing the correlation betweenthe optical band gap and the field effect mobility in the embodiment ofthe present invention;

FIG. 14 is a plan view showing the schematic construction of a generalliquid crystal display; and

FIG. 15 is a sectional view showing the structure of a conventional TFT.

DESCRIPTION OF THE PREFERRED EMBODIMENT

One embodiment of the present invention will be described below withreference to FIGS. 1 to 9.

FIG. 1 is a sectional view showing the structure of a TFT according tothe embodiment, and illustrates one example of the reverse stagger typestructure of a TFT.

In an illustrated TFT 1, as shown in FIG. 1, a gate electrode 3 made ofa metal such as aluminum is formed on a transparent substrate 2, and agate insulating film 4 made up of two layered insulating films is formedin covering relation to the gate electrode 3. A first gate insulatingfilm 5 and a second gate insulating film 6, which constitute the gateinsulating film 4, are each a film of silicon nitride (SiN_(x)). Thefirst gate insulating film 5 has a value of optical band gap in therange of 3.0 to 4.5 eV, and the second gate insulating film 6 has avalue of optical band gap in the range of 5.0 to 5.3 eV.

A semiconductor active film 7 made of amorphous silicon (a-Si) is formedon the gate insulating film 4 at a position above the gate electrode 3.A source electrode 9 and a drain electrode 10, each of which is made ofa metal such as aluminum, are formed to extend over the semiconductoractive film 9 through an ohmic contact layer 8 which is made ofamorphous silicon (a-Si:n⁺) containing an n-type impurity such asphosphorous, and then on the gate insulating film 4. A passivation film11 is formed in covering relation to the TFT 1 made up of the sourceelectrode 9, the drain electrode 10, the gate electrode 3, etc., and acontact hole 12 is formed in the passivation film 11 at a position abovethe drain electrode 10. Further, a pixel electrode 13 formed of atransparent conductive film, such as ITO, is filled in the contact hole12 for electrical connection to the drain electrode 10.

The procedures for producing a TFT array board using thethus-constructed TFTs 1 will now be described with reference to FIG. 2.

First, as shown in FIG. 2A, a conductive film is formed on thetransparent substrate 2 and then patterned to form the gate electrode 3and a gate line (not shown). Then, as shown in FIG. 2B, the gateinsulating film 4 made up of the two layered SiN_(x) films is formed incovering relation to the gate electrode 3. Thereafter, an a-Si film 14and an a-Si:n⁺ film 15 are formed successively in this order, and thesefilms 14, 15 are patterned together using one photomask, thereby formingan island portion 16 over the gate electrode 3 through the gateinsulating film 4.

A thin film forming apparatus used for forming the gate insulating film4 made up of the two layered SiN_(x) films will now be described.

FIG. 3 is a schematic view showing a two-frequency excitation plasma CVDapparatus 30 as a thin film forming apparatus according to theembodiment. In FIG. 3, numeral 31 denotes a plasma excitation powersupply, 32 denotes a radio-frequency (RF) electrode, 33 denotes a biaspower supply, 34 denotes a susceptor electrode, and 35 denotes a filmforming chamber.

The two-frequency excitation plasma CVD apparatus 30 comprises, as shownin FIG. 3, the film forming chamber 35 capable of maintaining a vacuumatmosphere in its inner space, the RF electrode 32 and the susceptorelectrode 34 for generating an RF electric field to produce plasmawithin the film forming chamber 35, the plasma excitation power supply31 and the bias power supply 33 for supplying respective RF powers ofpredetermined frequencies to the electrodes 32 and 34, a heating means(not shown) for heating a substrate 29 placed on the susceptor electrode34, a gas supply means 37 for supplying a desired gas to the inner spaceof the film forming chamber 35 through a gas introducing pipe 36, a gasdischarge means 38 for discharging the gas in match with the gas supplyto maintain a desired pressure within the film forming chamber 35, and acontrol unit 39 for controlling various components of the two-frequencyexcitation plasma CVD apparatus 30 including the heating means, theplasma excitation power supply 31, the bias power supply 33, the gassupply means 37, the gas discharge means 38, etc.

The two-frequency excitation plasma CVD apparatus 30 having theabove-described construction is operated in any of the following twotypes of sequences according to programs stored in the control unit 39.These two types of sequences are optionally selectable by an operator.

The first sequence comprises, as shown in FIG. 4, the steps of placingthe substrate 29 on the susceptor electrode 34 which is provided to facethe RF electrode 32 installed in the film forming chamber 35 (step S0 inFIG. 4); supplying a gas mixture of monosilane gas and ammonia gas tothe inner space of the film forming chamber 35 while exhausting the gasso as to maintain the desired pressure within the film forming chamber35, and bringing the gas mixture into a plasma state under a first RFelectric field formed between the RF electrode 32 and the susceptorelectrode 34, thereby forming the first gate insulating film 5 ofSiN_(x) on the substrate 29 (step S1 in FIG. 4); and bringing the gasmixture into a plasma state under a greater second RF electric fieldthan the first RF electric field while maintaining the plasma statebetween the RF electrode 32 and the susceptor electrode 34, therebyforming the second gate insulating film 6 on the surface of the firstgate insulating film 5 (step S2 in FIG. 4). The above steps are executedin squence to form the first gate insulating film 5 and the second gateinsulating film 6 successively.

A means for producing the second RF electric field greater than thefirst RF electric field in the above first sequence can be realized by amethod of setting the second substrate bias power applied to thesusceptor electrode 34 in forming the second gate insulating film 6 tobe greater than the first substrate bias power applied to the susceptorelectrode 34 in forming the first gate insulating film 5, or a method ofsetting the second plasma excitation power applied to the RF electrode32 in forming the second gate insulating film 6 to be greater than thefirst plasma excitation power applied to the RF electrode 32 in formingthe first gate insulating film 5.

The second sequence comprises, as shown in FIG. 5, the steps of placingthe substrate 29 on the susceptor electrode 34 which is provided to facethe RF electrode 32 installed in the film forming chamber 35 (step S0 inFIG. 5); supplying a first gas mixture of monosilane gas and ammoniagas, which are mixed at a first mixing ratio, to the inner space of thefilm forming chamber 35 while exhausting the gas so as to maintain thedesired pressure within the film forming chamber 35 (step S1′ in FIG.5); bringing the first gas mixture into a plasma state under an RFelectric field formed between the RF electrode 32 and the susceptorelectrode 34, thereby forming the first gate insulating film 5 ofSiN_(x) on the substrate 29 (step S1 in FIG. 5); and supplying a secondgas mixture of monosilane gas and ammonia gas, which are mixed at such asecond mixing ratio as containing the ammonia gas at a greaterproportion than at the first mixing ratio, to the inner space of thefilm forming chamber 35 while maintaining the plasma state between theRF electrode 32 and the susceptor electrode 34, and bringing the secondgas mixture into a plasma state to thereby form the second gateinsulating film 6 of SiN_(x) on the surface of the first gate insulatingfilm 5 (step S2 in FIG. 5). The above steps are executed in sequence toform the first gate insulating film 5 and the second gate insulatingfilm 6 successively.

Next, three concrete examples of forming the gate insulating film 4 madeup of two layered SiN_(x) films will be described.

As described above, the first gate insulating film 5 and the second gateinsulating film 6 are each a SiN_(x) film and can be formed successivelyusing the two-frequency excitation plasma CVD apparatus 30. However,since the two SiN_(x) films have different optical band gaps from eachother, they require to be formed separately.

The first example employs the above first sequence. In this method, asshown in FIG. 6, the substrate bias power applied to the susceptorelectrode 34 supporting the substrate to be processed thereon ischanged.

Concrete conditions for the film formation are as follows. The RFexcitation power of 40.68 MHz applied to the RF electrode is heldconstant at 600 W, whereas the substrate bias power of 13.56 MHz is setto 0 W (non-applied state) in forming the first gate insulating film 5and to 400 W in forming the second gate insulating film 6. During theprocess, the flow rate ratio of monosilane gas to ammonia gas, which arematerial gases for the SiN_(x) films, is held constant at NH₃/SiH₄:160sccm/40 sccm. The substrate temperature is held in the range of 250 to300° C., and the pressure within the chamber is held at 150 Pa.

The second example also employs the above first sequence. In thismethod, as shown in FIG. 7, the RF excitation power is changed.

Concrete conditions for the film formation are as follows. The substratebias power is held constant at 0 W (non-applied state), whereas the RFexcitation power is set to 200 W in forming the first gate insulatingfilm 5 and to 800 W in forming the second gate insulating film 6. Duringthe process, the flow rate ratio of monosilane gas to ammonia gas isheld constant at NH₃/SiH₄:160 sccm/40 sccm. The substrate temperature isheld in the range of 250 to 300° C., and the pressure within the chamberis held at 150 Pa.

The third example employs the above second sequence. In this method, asshown in FIG. 8, the flow rate ratio of monosilane gas to ammonia gas ischanged.

Concrete conditions for the film formation are as follows. The flow rateratio of monosilane gas to ammonia gas is set to NH₃/SiH₄:80 sccm/40sccm in forming the first gate insulating film 5 and to NH₃/SiH₄:240sccm/40 sccm in forming the second gate insulating film 6. During theprocess, the RF excitation power is held constant at 800 W and thesubstrate bias power is held constant at 100 W. The substratetemperature is held in the range of 250 to 300° C., and the pressurewithin the chamber is held at 150 Pa.

With any of the above three exemplified methods, it is possible toadjust a value of optical band gap of the first gate insulating film 5to fall in the range of 3.0 to 4.5 eV, and to adjust a value of opticalband gap of the second gate insulating film 6 to fall in the range of5.0 to 5.3 eV.

Returning to the process flow again, as shown in FIG. 2C, a conductivefilm is formed on the entire surface and then patterned to form thedrain electrode 10, the source electrode 9 and the source line (notshown). A part of the a-Si:n⁺ film 15 located above a channel portion ofthe a-Si film 14 is then removed to form the ohmic contact layer 8provided by the a-Si:n⁺ film 15.

Next, as shown in FIG. 2D, the passivation film 11 is formed on theentire surface and then patterned to open a part of the passivation film11 above the drain electrode 10, thereby forming the contact hole 12 forelectrical connection between the drain electrode 10 and the pixelelectrode 13. Finally, as shown in FIG. 2E, an ITO film is formed on theentire surface and then patterned to form the pixel electrode 13.Through the steps described above, the TFT array board of thisembodiment is completed.

In the TFT 1 of this embodiment, as described above, the gate insulatingfilm 4 is made up of the two layered SiN_(x) films having differentoptical band gaps from each other. Of these SiN_(x) films, the firstgate insulating film 5 functions to improve a withstand voltage betweenthe gate electrode 3 and the semiconductor active film 7, and the secondgate insulating film 6 functions to improve an interface characteristicbetween the gate insulating film and the semiconductor active film 7.Accordingly, looking at the gate insulating film 4 as a whole, a gateinsulating film can be realized which has in itself a desired dielectricwithstand voltage and renders the semiconductor active film 7 to have adesired carrier mobility. As a result, a TFT being less susceptible toan operation failure and capable of high-speed operation is achieved.

With any of the above three examples used as the method of forming thegate insulating film 4, the two layered gate insulating films 5, 6having different characteristics can be formed successively just bychanging over the excitation power, the substrate bias power or themixing ratio of the gas mixture at the time of shift from the first gateinsulating film forming step to the second gate insulating film formingstep. Thus, the TFT array board can be manufactured with productivity ata level not so lower than the case of forming a one-layer gateinsulating film.

One example of a TFT type liquid crystal display using the TFT arrayboard of this embodiment will be described below.

In the liquid crystal display of this embodiment, as shown in FIG. 9, apair of transparent substrates 2 and 17 are arranged in opposedrelation. Of these transparent substrates, one substrate 2 isconstituted by the TFT array board including the above-described TFTs 1,and the other substrate 17 serves as an opposite board. The pixelelectrode 13 is provided on a surface of the TFT array board 2 facingthe opposite board 17, and a common electrode 18 is provided on asurface of the opposite board 17 facing the TFT array board 2.Orientation films 19, 20 are provided on the pixel electrode 13 and thecommon electrode 18, respectively, with a liquid crystal layer 21disposed between the orientation films 19, 20. First and secondpolarizing plates 22, 23 are provided outside the transparent boards 2,17, respectively, and a backlight 24 is attached to the outer side ofthe first polarizing plate 22.

The TFTs 1 used in the TFT type liquid crystal display of thisembodiment each provide a high dielectric withstand voltage between thegate electrode 3 and the semiconductor active film 7, and ensure a greatcarrier mobility in the semiconductor active film 7. Therefore, a liquidcrystal display can be realized which has a high response speed and issuperior in yield and reliability. In addition, since the first gateinsulating film 5 of the TFT array, positioned on the same side as thebacklight 24 has a high dielectric withstand voltage and serves toabsorb ultraviolet rays, an adverse effect of light from the backlight24 upon the TFT characteristics can be minimized.

It is to be noted that the technical scope of the present invention isnot limited to the above-described embodiment, and the present inventioncan be modified in various ways without departing from the purport ofthe invention. For example, while the two-frequency excitation plasmaCVD apparatus is employed in forming the two-layered gate insulatingfilm in the above embodiment, the two-frequency excitation plasma CVDapparatus is not needed in the case of using a method of forming thefirst gate insulating film and the second gate insulating film withoutapplying the substrate bias power, and an ordinary plasma CVD apparatushaving only the RF electrode can be used. Further, while only threeexamples have been described as the methods of forming the gateinsulating film, any of other suitable methods such as described in“Summary of the Invention” is also usable.

<Examples>

The correlation between the optical band gap of the gate insulating filmand various characteristics in the TFT of the present invention wasstudied. Results of the studies are reported below.

First, how the optical band gap varies depending on values of theapplied RF power in forming the gate insulating film was studied. Usingthe two-frequency excitation plasma CVD apparatus, SiN_(x) films wereformed while the RF excitation power (Rf₁) of 40.68 MHz was changed tofour values, i.e., 200, 400, 600 and 800 arb. (arb. represents anarbitrary unit) and the substrate bias power (Rf₂) of 13.56 MHz waschanged from 0 arb. to 400 arb. for each value of the RF excitationpower. The optical band gap of each of the formed SiN_(x) films wasmeasured. As other conditions for the film formation, the flow rateratio of material gases was set to SiH₄/NH₃/N₂:40 sccm/160 sccm/600sccm, the substrate temperature was set to 300° C., and the pressurewithin the chamber was set to 250 Pa. Measured results are plotted inFIG. 10.

In FIG. 10, the horizontal axis represents the substrate bias power(arb.) during the formation of each film, and the vertical axisrepresents the optical band gap (eV) of each film. Also, marks “∘” inFIG. 10 denote data resulted at the RF excitation power (Rf₁) of 200arb., marks “▪” denote data resulted at Rf₁ of 400 arb., marks “▴”denote data resulted at Rf₁ of 600 arb., and marks “∇” denote dataresulted at Rf₁ of 800 arb.

It was found from the results of FIG. 10 that when the RF excitationpower (Rf₁) was set to 200 arb., the optical band gap of the SiN_(x)film took a value in the range of 3.5 to 4.5 eV by forming the film withthe substrate bias power (Rf₂) set to a value in the range of 0 to 200arb.

Next, SiN_(x) films were formed by setting the RF excitation power (Rf₁)to 0, 100, 200 and 300 arb. while the substrate bias power (Rf₂) waschanged from 0 arb. to 500 arb. The dielectric withstand voltage wasmeasured for each of the formed SiN_(x) films.

In the experiment, the dielectric withstand voltage was measured inaccordance with two patterns, i.e., a “flat-portion withstand voltagemeasurement pattern” in which a first conductive film with a thicknessof 1300 Å, a SiN_(x) film with a thickness of 1300 Å and a secondconductive film with an arbitrary thickness were stacked on a substratein a sectional structure, and the dielectric withstand voltage wasmeasured in a flat portion of the SiN_(x) film between the first andsecond conductive films each having a predetermined area and beingrectangular in plan view; and a “stepped-portion withstand voltagemeasurement pattern” in which the first and second conductive films wereeach formed linearly in crossed relation, and the dielectric withstandvoltage was measured in a stepped (level-difference) portion of theSiN_(x) film where the first and second conductive films cross eachother. FIG. 11 shows measured results of the flat-portion dielectricwithstand voltage, and FIG. 12 shows measured results of thestepped-portion dielectric withstand voltage.

In FIGS. 11 and 12, the horizontal axis represents the applied RF power(arb.: the term “applied RF power” means the total of the RF excitationpower (Rf₁) and the substrate bias power (Rf₂)) during the formation ofeach film, and the vertical axis represents the dielectric withstandvoltage (MV/cm). In the measurement, the RF excitation power (Rf₁) wasset to 0, 100, 200 and 300 arb. In both FIGS. 11 and 12, marks “”denote data resulted at the RF excitation power (Rf₁) of 0 arb., marks“Δ” denote data resulted at Rf₁ of 100 arb., marks “▪” denote dataresulted at Rf₁ of 200 arb., and marks “∘” denote data resulted at Rf₁of 300 arb.

In the measurement of the flat-portion dielectric withstand voltageshown in FIG. 11, the dielectric withstand voltage was about 2 MV/cmwhen the applied RF power was 0 arb., and increased to about 8 MV/cm asthe applied RF power was increased to 500 arb. On the other hand, fromthe measurement of the stepped-portion dielectric withstand voltageshown FIG. 12 which represents more strict evaluation of the dielectricwithstand voltage, it was found that the dielectric withstand voltagehad values in the range of 6 to 8 MV/cm when the applied RF power was inthe range of 0 to 200 arb., but reduced down below 1 MV/cm as theapplied RF power was increased to 500 arb., whereby the desiredwithstand voltage was not obtained.

Considering the withstand voltage of the gate insulating film in a TFTof the reverse stagger type, in particular, the gate insulating film isnecessarily stepped at the end of a gate electrode, and breakdown of thegate insulating film is more apt to occur due to concentration of anelectrical field in such a stepped portion. From the viewpoint ofensuring the normal operation of the TFT, it is therefore important thatthe dielectric withstand voltage is high particularly at the steppedportion.

For that reason, the applied RF power in the process of film formationis preferably held in the range of 0 to 200 arb. Judging from theresults of FIGS. 10 to 12 comprehensively, it was proved that when theoptical band gap of the SiN_(x) film had a value in the range of 3.5 to4.5 eV, the SiN_(x) film showed a satisfactory dielectric withstandvoltage, particularly in the stepped portion of the gate electrode, anda TFT having high reliability was obtained.

Further, several SiN_(x) films having different optical band gaps fromeach other were formed, and the field effect mobility was measured usingeach of the formed SiN_(x) films. Two types of samples were prepared bychanging the pressure within the chamber in the process of forming theSiN_(x) film (marks “” in FIG. 13 denote the samples prepared at thepressure P₁Pa and marks “▪” in FIG. 13 denote the samples prepared atthe pressure P₂Pa), and were subject to measurement of the field effectmobility. Measured results are shown in FIG. 13.

In FIG. 13, the horizontal axis represents the optical band gap (eV) ofeach film, and the vertical axis represents the field effect mobility(cm²/V·sec). It was found from the results shown in FIG. 13 that whenthe optical band gap is in the range of 3.5 to 5.0 eV, the field effectmobility has a tendency to gently increase with an increase of theoptical band gap. On the other hand, when the optical band gap exceeded5.0 eV, the field effect mobility started to increase abruptly, and whenthe optical band gap reached 5.3 eV, the field effect mobility had avalue of approximately of 1.0 cm²/V·sec that was about twice the valueresulted when the optical band gap was 3.5 eV. It was thus proved thatusing the SiN_(x) film having the optical band gap in the range of 5.0to 5.3 eV is preferable from the viewpoint of improving the carriermobility.

According to the present invention, as described above in detail, thefirst gate insulating film functions to improve a withstand voltagebetween the gate electrode and the semiconductor active film, and thesecond gate insulating film functions to improve an interfacecharacteristic between the gate insulating film and the semiconductoractive film. Therefore, looking at the gate insulating film as a whole,a gate insulating film can be realized which has in itself a desireddielectric withstand voltage and renders the semiconductor active filmto have a desired carrier mobility. As a result, a TFT being lesssusceptible to an operation failure and capable of high-speed operationis achieved. Further, with the method of forming the gate insulatingfilm according to the present invention, the two layered gate insulatingfilms having different characteristics can be formed successively justby changing over the excitation power, the substrate bias power or themixing ratio of the gas mixture at the time of shift from the first gateinsulating film forming step to the second gate insulating film formingstep. Thus, the TFT array board can be manufactured with productivity ata level not so lower than the case of forming a one-layer gateinsulating film. Moreover, with the thin film forming apparatusaccording to the present invention, the excitation power, the substratebias power or the mixing ratio of the gas mixture can be changed fromone level to the other level at the time of shift from the first gateinsulating film forming step to the second gate insulating film formingstep, and the two layered gate insulating films having differentcharacteristics can be formed successively. It is thus also possible tomanufacture the TFT array board with productivity at a level not solower than the case of forming a one-layer gate insulating film. Inaddition, according to the present invention, a liquid crystal displaycan be realized which has a high response speed and is superior in yieldand reliability.

What is claimed is:
 1. A thin film transistor wherein a gate electrodeand a semiconductor active film are formed on a substrate with a gateinsulating film, which is formed of two layered insulating films,located therebetween, said gate insulating film being made up of a firstgate insulating film which is disposed on the same side as said gateelectrode and improves a withstand voltage between said gate electrodeand said semiconductor active film, and a second gate insulating filmwhich is disposed on the same side as said semiconductor active film andimproves an interface characteristic between said gate insulating filmand said semiconductor active film, wherein said first and second gateinsulating films are each formed of a silicon nitride film, the opticalband gap of said first gate insulating film has a value in the range of3.0 to 4.5 eV, and the optical band gap of said second gate insulatingfilm has a value in the range of 5.0 to 5.3 eV, and the first gateinsulating film has a dielectric withstand voltage in the range of 6 to8 MV/cm.
 2. A liquid crystal display wherein a liquid crystal is heldbetween a pair of substrates disposed in opposed relation, and one ofsaid pair of substrates includes a thin film transistor according toclaim
 1. 3. A thin film transistor comprising: a substrate; a gateelectrode formed on the substrate; a semiconductor active film over thegate electrode; and a layered gate insulating film formed between thegate electrode and the semiconductor active film, the layered gateinsulating film including a first gate insulating film having an opticalband gap of 3.0 to 4.5 eV and a dielectric withstand voltage of 6 to 8MV/cm and a second gate insulating film of 5.0 to 5.3 eV.
 4. A liquidcrystal display wherein a liquid crystal is held between a pair ofsubstrates disposed in opposed relation, and one of said pair ofsubstrates includes the thin film transistor according to claim 3.